Amplification system



April 22, 1930. 1 THOMPSON y AMPLIFICATION SYSTEM Filed May` l, 1928 2Sheets-Sheet l llllll -ill o Y R om m T .0. R mh No VT n m n, A

.,Pmntjed Apr. 22', 1930 l .UNITED STATES .PATENT OFFICE LINCOLNTHOMPSON, .OF NUGATUCK, CONNECTICUT, ASSIGNOR, BY MESNE ASSIGN- A1t1ENTS, T0 WM.r H. BRISTOL TALKING PICTURE CORPORATION, OF WATEBBURY,

CONNECTICUT, A. CORPORATION OF CONNECTICUT AMPLIFICATION SYTEMApplication mea nay 1, 192s. Serin No. 274,174.

i IThe invention'relates to an amplifying sys-r tem for weak electriccurrents and more especially to a system utilizing thermionlc valves orso-called audion tubes for this purpose.

q In the amplification of these currents as heretofore conductedrovision has been made to render the amplification "as distortionless asp ossible, more especially through the prevention of any aplpreciablefiow of current in the grid circuit, t us also insuring a minimumrequirementl of energy from the applied vsignal or other energizingsource to be amplified.

This substantially distortionless amplification has been accomplished byimpressing on the grid of an amplifying valve a potential permanentlymore negative than that of its cathode, 'or biasing negatively saidgrid. The magnitude of this bias or negative potential dependson thegrid potential-plate current characteristic of the particular tubeemployed, it being understood that the amount of grid current will benegligible until the signal voltage attains a value sufficient to makethe grid reach or exceed the filament potcntial. Actually, appreciablegrid current fiows at a slightly negative potential, the current thenflowing introducing a shunt load on the signal source and, in theordinary well-known amplification circuit causing distortion of theinput signal wave form which is correspondingly magnified in theamplified circuit. If the valve, however, were operated withoutproviding the aforesaid grid bias, the entire positive, pulsations ofthe signal would be subject `to this shunt load and the negativepulsations be subjected to practically no load,

the distortion, therefore, being especially severe.

Vacuum tube design has, therefore, been along the lines of constructingtubes such that their characteristic curve will show as great a usefulportion to the negative side of the zero grid potential line as possiblein order to give Imaximum undistorted output. As a matter of fact, sometubes in use have no useful portion on the positive side. This, however,entails a loss' of amplifying power of the tube, that is to say, in thenumber of milliamperes change of the plate current for a unit` v changein the grid voltage and therefore less steep gradient. If it werepossible to operate the tubes without this distortion, using tubes whosecharacteristic curve shows a much steeper gradient, with useful portionssubstantially equal' on both sides of the zero grid potential line, avery much smaller signal voltageiwould have to be applied to equal theoutput of the biased tube, or a much gater amplification ratio would beattaina e.

The present invention has for its object a novel system whereby thisincreased amplification ratio is secured, it being possible to operate atube under substantially distortionless amplification and with the tubehaving no bias whatsoever or otential impressed permanently on its grior, if any be thus permanently impressed thereon, the same being apositive one. I accomplish this by a novel system or method of balancingwhich includes setting up positive grid currents in phase with theoscillating voltage applied to the input circuit of the tube and therebyrendering their effect on distortion negligible. This entails nosacrifice of amplification ratio, although the grid circuits consumeappreciable energy in their operation yet this is more than compensatedfor by the increased amplifying power. By properly designing thecircuits to allow for the grid energy consumption, excellent quality isattained and at tremendous volumes.

A further object of the invention contem-l plates a system wherein gridleaks may be dispensed with, provision being made for the 'discharge ofany accumulated potential through circuits employed in the operation ofvthe novel system. l

rent ioWing therein is balanced or com ensated so that distortion issubstantially o viated and a high amplification ratio attained.

' trical 'communication with the cathode of the fication constant, andembodying also the other tube, while the grid ofthe latter is similarlyindirect electrical communication with the cathode of the first-namedtube and both the said grid and cathode of the lastnamed tube beingindirectelectrical communication with the input circuit, whereby thecombined grid circuits function like an equivalent ohmic resistanceplaced across the input circuit-alternating positive and negatlveportions of the applied signal current of the input circuit equally andin accordance with the applied voltages.

The nature of the invention, however, will best be understood whendescribedjn connection with the accompanying drawings, iny

which:

Fig. 1 is a diagrammatic view illustrating the novel amplificationsystem as applied in connection with a microphone.

Fig. 2-illust'rates one arrangement for combining the outputs of the twothermionic valves of thesystem illustrated in Fig. 1.

Fig. 3 is a diagrammatic representation of the novel amplifying systemembodying a modification.

Fig. 4 is a diagrammatic view of the novel ampllfying system connectedwith, -for l example, a detector tube or lower amplification stage.

Figs. 5 and 6 are similar views illustrating7 different means ofcoupling.

igs. 7 and 8 are diagrammatic views of the amplifying system showing amodification in the compensating arrangement.

Fig. 9 illustrates the arrangement for feeding the filaments of thetubes employed in the novel amplifying system from a source ofalternating current.

Referring to Fig. 1 of the drawings, an input circuit is indicated, 10designating a suitable Vinductance thereof receiving a single voltage inanywell-known or special manner and from which inductance extends a lead11 connected directly to the grid 12 of an amplifying tube 13,preferably of so-called hardor highly evacuated type and of highampliplate or anode 14 andthe lament or cathode 15. A further lead 16directly connects the latter to said inductance; and the lament orcathode 15 and plate or anode 14 are connected in the usual manner tothe A and B batteries 17 and 18 respectively in ac plifying circuitexcept that no C battery is included in circuit for biasing negativelythe grid 12, nor is there provided any gridleak in connection therewith.The novel system, operates best when preceded by a signal:

practical arrangement for carrying out,

source of low impedance, for example, a carbon microphone 19 which feedsthe in ut circuit, through the inductance 10 mate ed thereto,excellently because ofits ability to deliver current even at fairlyhighsteppedup voltage.

In accordance with the invention, however, there is associated with thetube 13 a similar tube 20 embodying the grid 21, anode 22 and cathode23; andthe grid 12 of the former tube is directly connected with thecathode 23 of the latter tube by means of a lead 24, while its grid 21in turn is directly connected to the cathode 15 of the tube 13 by meansof a lead 25. Moreover, it will be noted that both the grid 21 and itscathode 23 are thereby also directly connected to the input circuit.Means, independent of the filament and vplate energizing batters 17 and18, are also to ybe provided for the cathode 23 and anode '22 of thissecond tube, for example A and B batteries 26 and 27 respectively.

" It will be noted that, if the am liying arone tube 13, this mayreadily be effected by including in series in the lead 11 a C battery 28or like expedient, Fig. 3, for impressing permanently on the said gridthe desired potential which, it will be understood, will always be morepositive than the potential of the corresponding cathode 15 of saidtube.

This provision of a lpositive bias is more especially valuable in theleading stages of ampliication, rather than in the final or powerstages.

In the event of the input circuit having a condenser 29, Fig. 3, inseries in the lead 11 the grid current of each tube under staticconditions may be considered as returning to its filament through thegrid filament path of the other tube. The oscillating grid currents setup, however, may pass through the capacity and the applied circuit. v

When the novel amplifier circuit is lto be coupled to a vacuum tube, asthe tube 30, Fig. 4, this tube should be of fairlylow impedance and thecoupling may be effected in any of the customary ways. For example, asindicatedin Fig. 4, an impedance 31 may be bridged across the platecircuit of said tube to carry oHthe direct current, while a condenser 32is inserted in series in the lead 33 therefrom to allow the alternatingvoltages impressed on the grid circuit of the tube to Abe conitinued tot-he amplifyin portion com- ,the plate circuit of the tube 3 while inFig.

6, a typical transformerv couplin is shown, the primary of transformer38 bem bridged across the plate circuit of tube 39 an matched .to its.late im edance, while-the secondary thereo is matc ed to the gridimpedance of the grid circuit to attain maximum efliciency. lThematching is designed for the desired frequency,.say a proximately 800cycles.

It will be o served that no grid leaks are included in the foregoingembodiments as the same are not required, for a grid circuit of one tubeis free to discharge through the grid circuit of the other tube of saidpair of ainplifying tubes.

Reference being had to Fig. 7, this balancing or compensation may beaccomplished, also, b disconnecting entirely the p ate circuit o one ofthe tubes as the tube 40, or utilizing a plateless tube, and including avariable resistance element 41 in the grid lead 42 from the grid of theother tube 43. This resist-ance is to be adjusted to give equalgrid-current grid-voltage curve with the main amplifier tube- 43, as theusual two-.element tube does not possess generally the 4desiredcharacteristic for'matching the grid-current grid-voltage ofsaid tube43. Or a two-element tube or its equivalent such as a crystal or otheruni-lateral device (not shown) whose voltage-current curve can beadjusted to equal more or less exactly thevoltage-current curve of theamplifier tube may be utilized for this purpose.

Means may be devised to reduce the grid current, for example, by placingin series the grid circuits of two or more tubes 46 and 47 combined witha tube 48, Fig. 8, and compensating as hereinbefore noted, as with theva'- riable resistance element 49 and plateless tube 48. v Moreover, thefilaments or cathodes are vadapted for energization from a source ofthiiough a suitable transformer 53 and the returii lead 54 of the onegrid 5 5 is not required to be made to return to the electrical centerof the other tube for minimized hum, thus eliminating the necessity forcenter taps in 'the :transformer winding. This results from the eti'irnto the proper side of the windings (taking into account phase relation)of the one grid which receives a positive pulsation while the otherneutralizes it with a negative pulsation. Furthermore, the low impedancegrid circuits make said circuits less susceptible to picking up hum. y

1t will be understood that the resistance of -the novel grid circuitshereinbefore described is much lower than is the case with negativelbiased amplifier circuits; and some loss is thereforeY involved, but bycareful selection of the filament and plate voltages this loss can beminimized.

The general operation of the improved amplifier system is as follows:

Referring to Fig. 1, it is obvious that the the grid 12, for example,being positive with respect to lead 16, an electron current flows in thetube 13 from its filament or cathode 15 to to the said grid 12 throughthe lead 11 ofthe input circuit and lead 16 thereof back to saidfilament. At this instant, the grid 21 of tube 20 is negative withrespect to its filament 23 and no current can flow from thisfilament tothe grid 21. Conversely, when lead 16 and the connected grid 21 arepositive with respect to the grid 12 and connected lead 11 of the inputcircuit, current flows from the filament 23 to its grid butno current'can flow in the tube 13 between its filament 15 and grid 12, which areboth negative at this instant.

An oscillating voltage applied to the input circuit will, therefore, setup oscillating currents in the interconnected grid filament circuit ofthe two tubes and in phase with the applied oscillating voltage. Thesaid interconnected circuit will then act as an ordinary ohmicresistance across the input circuit 11-16, alternating positive andnegative portions'of the applied signal equally since the tubes 13 and2() are designed to match.

As the curve of grid-current against gridvoltage is, in actual operationofthe circuit, substantially a straight line within the working limits,equal increases of voltage will result substantially in equal increasesof the grid current so that reverse currents will result from reversevoltages and in equal 'amounts for equal variations thereof. 4

Moreover, in the use of an amplifying system employing one or morestages ot ampli- .ficatioin the preceding` stages in a system of thenature herein set forth may be operated at positive or zero potential;and it has been .found generally advantageous to operate one `or moreleading stages at a predetermined positive bias. Since the variable gridimped-,- ance is the factor which usually produces distortion invamplitying systems, there is sel lected in the present embodiment apoint in the grid-voltage grid-current curve at which thisgridimpedance, although low, is conl stant. The said curve is substantiallya iis isc

straight line over a' definite positive portion; and the biasing pointselected is to be located substantially midway between the zero voltageposition and the point where said curve begins to depart from itsstraight line characteristic. This will permita tube to handle withoutdistortion fairly large variations in voltage abut when the compensatedarrangement is to be provided as in larger amplilications, as the finalor power stages of a system, the grid is to be operated at substantiallythe zero voltage position-fthe combined grid impedancespf the two tubesemployed being equal only to that of one tube.

In connection with the operation at a point along'the curve, namely apredetermined positive bias, it will be appreciated that both a ositiveand negative variation of the signal beyond the bias portion will beaffected substantially alike so thatf'no serious distortion effects willresult. With -this system of operation, therefore, -a steady flow ofcurrent prevails in the grid circuit under static conditions, which'current increases and decreases in value with the input signal; and thepositive bias therein makes for lower operating` plate impedance of thetube.

When positively biased, a lower operating plate impedance of the tuberesults, making the same ideal for preceding a compensated stage ofamplification. Y

' Moverover, when a coupling transformer is employed between stages, theflow of direct currentvin the plate winding thereof tends to saturatethe transformer core; but when connected with a positively biased tubeof the character hereinbefore described, the direct current flowingthrough the grid may be applied in such a direction as to counteract theeffect of the aforesaid plate current of the tube of a preceding stageof amplification and thus prevent saturation and obviate the necessityfor a bulky ,transformer core.

I claim:

1. In an amplifying system having a thermionic valve with controllingelement, and anode and cathode included in a suitably energized circuit,and an input circuit connected to the said controlling element andcathode: a unilateral device included in the said input circuit, havingits voltage-current characteristic adjusted to match the grid-voltagegridcurrent characteristic of said thermionic valve and thereby todirect current through said unilateral device such that equal positiveand negative voltages applied to the input circuit cause substantiallyequal iiows of current respectively between said cathode and the saidcontrolling element and through the said unilateral device.

2. In an amplifying system having a thermionic valve with controllingelement, and anode and cathode included in a suitably energized circuit,and an input circuit connected to the said controlling element andcathode: a unilateral device included inparallel with said inputcircuit, having its 'voltage current characteristic adjusted to matchthe that equal positive and negative voltages aplied to the inputcircuit cause substantialy equal flows of current respectively betweensaid cathode. and the said controlling element and through theV saidunilateral device.

3. In an amplifying system having a thermionic valve with controllingelement, and anode and cathode included in a suitably energized circuit,and an input circuit connected to the said controlling element andcathode: a second thermionic valve with controlling element, and anodeand cathode included in a suitably energized circuit, the controllingelement thereof between connected to the said input circuitfto directcurrent between the last-named cathode and last-named controllingelement such that equal positive and negative voltages applied to theinput circuit will cause substantially e ual Hows of currentrespectively between t e first-named cathode and its controlling elementand the lastnamed cathode and its controlling element. 4. In anamplifying system having a thermionic valve with controlling element,and

- anode and cathode included in a suitably energized circuit, and aninput circuit connected to the said controlling element and cathode:means for impressing upon the controlling element of said valve apotential more positive than that of its cathode; and a unilateraldevice included in the said input circuit, having its voltage-currentcharacteristic adjustedto match the grid-voltage gridcurrentcharacteristic of said thermionic valve and thereby to direct currentthrough said unilateral device such that equal positive and negativevoltages applied to the input circuit cause substantially equal flows ofcurrent respectively between said cathode and the said controllingelement and through the said unilateral device.

5. In an amplifying system having a thermionic valve withcontrollingelement, and anode and cathode included in a suitably en'-ergized circuit, and an input circuit connected to the said controllingelement and cathode a second thermionic valve with controlling element,and anode and cathode included in a suitably energized circuit, thecontrolling element thereof' being connected to the said input circuitand to the first-named cathode and the cathode thereof to thefirst-named controlling element to direct current between the last-namedcathode and last-named controlling element such that equal positive andnegative voltages applied to the input circuit will cause substantiallyequalflows of currentv respectively between the rst-named cathode andits controlling element and the being connected to the cathode of theother and the said controlling elements and cathodes to the inputcircuit.

7. In an amplifying system having a thermionic valve with controllingelement, and

anode and cathode included in a suitably energizedcircuit, and an inputcircuit; means for impressing upon the Controlling element of said valvea. potential more positive than that of its cathode; and a secondthermionic valve with controlling element, and anode and cathodeincluded in a suitably energized circuit, the grid-current grid-voltagecharacteristic of the second valve being matched to the grid-currentgrid-voltage characteristi'c of the first-named valve and thecontrolling element of each valve being connected to the cathode of theother and the said controlling elements and cathodes to the inputcircuit.

In testimony whereof I aix my signature.

LINCOLN THOMPSON.

CERTIFICATE or CORRECTION.

-Patent No. 1,755,938. E

Granted April 22, 1930, to

i LINCOLN THOMPSON.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,line 45, for the word "single" read "signal"; line .84, for the word"batters" read "batteries", and line 88, for the word "if" read "in";page 4, line 85, claim 3, for the word "between" read "being"; and thatthe said Letters Patentshould be read with these corrections thereinthat the same may conform to the record of the case in the PatentOffice.

Signed and sealed this 27th day of May, A. D'. 1930.

" (seal) y M. J. Moore, 1 Acting Commissioner of Patents.

